[build]: Add missing timestamp and build_number in docker meta label tag value (#2194)

* Add missing timestamp in docker meta label tag value

Signed-off-by: Jipan Yang <[email protected]>

* add BUILD_NUMBER as well

Signed-off-by: Guohan Lu <[email protected]>

Author: jipanyang

slave.mk

@@ -9,7 +9,7 @@ SHELL = /bin/bash
 USER = $(shell id -un)
 UID = $(shell id -u)
 GUID = $(shell id -g)
-SONIC_GET_VERSION=$(shell . functions.sh && sonic_get_version)
+SONIC_GET_VERSION=$(shell export BUILD_TIMESTAMP=$(BUILD_TIMESTAMP) && export BUILD_NUMBER=$(BUILD_NUMBER) && . functions.sh && sonic_get_version)
 
 .SECONDEXPANSION: